Although we cannot see a molecule of a substance with our naked eyes, these minute particles do have a particular shape or geometry which greatly influences the observed properties of the substances.
Molecular geometry or molecular structure is the three-dimensional arrangement of atoms within a molecule. It is important to be able to predict and understand the molecular structure of a molecule because many of the properties of a substance are determined by its geometry.
The Valence Shell, Bonding Pairs, and VSEPR Model
The outermost electrons of an atom are its valence electrons. The valence electrons are the electrons that are most often involved in forming bonds and making molecules.
Pairs of electrons are shared between atoms in a molecule and hold the atoms together. These pairs are called "bonding pairs".
One way to predict the way electrons within atoms will repel each other is to apply the VSEPR (valence-shell electron-pair repulsion) model. VSEPR can be used to determine a molecule's general geometry.
Predicting Molecular Geometry
Here is a chart that describes the usual geometry for molecules based on their bonding behavior. To use this key, first draw out the Lewis structure for a molecule. Count how many electron pairs are present, including both bonding pairs and lone pairs. Treat both double and triple bonds as if they were single electron pairs. A is used to represent the central atom. B indicates atoms surrounding A. E indicates the number of lone electron pairs. Bond angles are predicted in the following order: lone pair versus lone pair repulsion > lone pair versus bonding pair repulsion > bonding pair versus bonding pair repulsion.
You can predict the molecular geometry of molecules using the following steps:
Step 1. Draw the appropriate Lewis Structure.
Step 2. Determine the number of electron groups around the central atom and
identify each as bonding pair or lone pair.
Step 3. Determine the Molecular Geometry from the table below.
Molecular Geometry
Geometry
Number of Electron Groups
Number of Lone Pairs, Bonding Pairs
Formula
Bond Angle
Example
Linear
2
0,2
AX 2
180°
BeCl 2
Trigonal Planar
3
0,3
AX3
120°
BF3
Angular
3
1,2
AX2
120°
SO2
Tetrahedral
4
0,4
AX4
109.5°
CH4
Trigonal Pyramidal
4
1,3
AX3
107°
NH3
Angular
4
2,2
AX2
104.5°
H2O
Trigonal Bipyramidal
5
0,5
AX5
90°,120°,180°
PCl5
Seesaw
5
1,4
AX4
90°,120°,180°
SF4
T-shaped
5
2,3
AX3
90°
ClF3
Linear
5
3,2
AX 2
180°
XeF2
Octahedral
6
0,6
AX6
90°,180°
SF6
Square Pyramidal
6
1,5
AX5
90°
BrF5
Square Planar
6
2,4
AX4
90°
XeF4
Youtube Video About Molecular Geometry
Pictures of Molecular Geometry
A. Linear
B. Trigonal Planar
C. Trigonal Planar: Angular(Bent)
D. Tetrahedral
E. Tetrahedral: Trigonal Pyramidal
F. Tetrahedral: Angular(Bent)
G. Trigonal Bipyramidal
H. Trigonal Bipyramidal: Seesaw
I. Trigonal Bipyramidal: T-shaped
J. Trigonal Bipyramidal: Linear
K. Octahedral
L. Octahedral: Square Pyramidal
M. Octahedral: Square Planar
Five Basic Geometrical Shapes:
Two Electron Pairs (Linear)
The basic geometry for a molecule containing a central atom with two pairs of electrons is linear. BeF2 is an example. Another example of a linear compound is CO2. However, its Lewis structure contains two double bonds. We need to recognize that multiple bonds should be treated as a group of electron pairs when arriving at the molecular geometry.
Three Electron Pairs (Trigonal Planar)
The basic geometry for a molecule containing a central atom with three pairs of electrons is trigonal planar. BF3 is an example. If we replace a bonding pair with a lone pair, as in SO2, the geometry is described as bent or angular.
Four Electron Pairs (Tetrahedral)
The basic geometry for a molecule containing a central atom with four pairs of electrons is tetrahedral. An example of this geometry is CH4. As we replace bonding pairs with nonbonding pairs the molecular geometry become trigonal pyramidal (three bonding and one nonbonding), bent or angular (two bonding and two nonbonding) and linear (one bonding and three nonbonding). Notice that compounds with the same number of terminal atoms, BF3 and NF3, do not necessarily have the same geometry. In this case BF3 has three bonding pairs and no nonbonding pairs with a geometry of trigonal planar, while NF3 has three bonding pairs and one nonbonding pair with a geometry of trigonal pyramidal. Also note that SO2 and H2O have a similar descriptor for their respective geometry. Although each molecule can be described as having a bent geometry the respective bond angles are different. For SO2 the O-S-O angle is near 120 degrees, actually slightly less than 120, about 118 degrees, for H2O the H-O-H angle is near 105 degrees.
Five Electron Pairs (Trigonal Bipyramidal)
The basic geometry for a molecule containing a central atom with five pairs of electrons is trigonal bipyramidal. An example of this geometry is PCl5. As we replace bonding pairs with nonbonding pairs the molecular geometry changes to seesaw (four bonding and one nonbonding), T-shaped (three bonding and two nonbonding) and linear (two bonding and three nonbonding). This is an interesting system because of the two different types of terminal atoms in the structure, axial and equitorial. The equitorial terminal atoms are those in the trigonal plane. The axial atoms are those above and below the trigonal plane. When the first bonding pair of electrons is replaced with a nonbonding pair that occurs in the trigonal plane. the reason for this is due to the smaller replusions between the lone pair and the bonding pairs of electrons. If the lone pair replaced an axial atom the repulsions would be greater. So as the bonding pairs of electrons are replaced with nonbonding pairs the equitorial atoms are replaced. So as we move from trigonal bipyramidal to linear the nonbonding pairs of electrons occupy the equitorial plane, not the axial positions.
Six Electron Pairs (Octahedral)
The basic geometry for a molecule containing a central atom with six pairs of electrons is octahedral. An example of this geometry is SF6. As we replace bonding pairs with nonbonding pairs the molecular geometry changes to square pyramidal(five bonding and one nonbonding) to square planar (four bonding and two nonbonding). There are no other combinations of bonding groups and nonbonding pairs of electrons when the electron-pair geometry is octahedral. The replacement of the first bonding group can occur in any position and always produces a square pyramidal molecular geometry. However the seond bonding group replaced is always opposite the first producing the square planar molecular geometry.
